The present invention relates to a system for reducing harmonic currents in an electric power conversion system where three-phase alternating current (AC) power is converted to direct current (DC) power and vice versa, and, more particularly, to a system that modulates current on the DC portion of the conversion system to create a selected harmonic current that is injected into the AC portion of the conversion system.
Converter systems and inverter systems are used throughout electric utility power systems to transfer power from and to the utility system grid, respectively. Converter systems transform alternating current power from the utility system to direct current power as a front end in loads such as uninterruptable power supplies (UPS), adjustable speed drives, induction heaters and arc welders. In contrast, inverter systems transform direct current power provided from renewable energy sources such as wind, photovoltaic or small hydro-powered generating systems to alternating current power that is provided to the electric utility system. In other energy storage systems such as batteries and magnetic storage, a combination of a converter and an inverter allows power to flow bidirectionally rather than unidirectionally.
However, as is commonly known, each of the aforementioned converter and inverter systems also contribute harmonic currents into the utility system grid. These harmonic currents may distort the generated voltage waveform at the point of common coupling with the power conversion system due to the finite system impedance of the utility system grid. In addition to voltage waveform distortion, harmonic currents interfere with communication and control signals, cause economic losses due to errors in metering and malfunctioning of utility system protection relays and stress the utility system equipment from heat generated by the harmonic currents and over-voltage conditions that occur in resonant situations.
Due to the increased use of converter and inverter conversion systems, and the inherent problems associated with the harmonic currents that they produce, national and international agencies have established various standards and guidelines to specify allowable limits for harmonic currents on a system. Some of these standards include: EN 50 006, "The Limitation of Disturbances in Electricity Supply Networks Caused by Domestic and Similar Appliances Equipped with Electronic Devices," European standard prepared by Comite Europeen de Normalisation Electrotechnique, CENELEC; IEC Norm 555-3, prepared by the International Electrical Commission and IEEE "Guide for Harmonic Control and Reactive Compensation of Static Power Converters", ANSI/IEEE standard 519-1981, which is expected to be revised.
For example, the revised IEEE-519 standard will contain recommended practices and requirements for harmonic control in the electric power system by specifying restrictions on the power conversion user as well as on the utility. Specifically, this standard will include upper limits on the amount of harmonic currents that a power conversion system user may inject into the utility system, and will provide voltage quality requirements upon the utility, provided that the harmonic currents injected by the user are within the specified limits.
In view of the standards imposed, various techniques have been implemented to reduce the harmonic currents present onto the utility power system. These techniques include passive filtration, active filtration and current wave shaping systems. In passive filtration, a fixed filter is connected to the utility system at the point of common coupling. The passive filter provides a low impedance path for the generated harmonic currents, thus bypassing them from entering the utility system. Significant drawbacks are present with this filtering technique. For example, since the passive filter is in parallel with the utility system impedance, a resonance condition may result that could cause an over-voltage condition at the point of common coupling. In addition, besides "shunting" or "sinking" harmonic currents generated from the power conversion system, the passive filter further sinks harmonic currents generated elsewhere in the utility system. Consequently, the power ratings of the passive filter components must be increased to handle the additional load requirements. These high power components raise the cost of the filter.
Active filters are also connected to the alternating current power utility system when they are used. When active filters are used, harmonic currents generated by the power conversion system (from AC-DC or reverse) are measured on the utility system. The active filter includes a switch-mode power electronics converter that supplies the harmonic currents drawn by the power conversion system so they are not emanating from the utility system. However, since the harmonic currents can be almost as large in magnitude as the fundamental frequency current, the power rating of the active filter approaches that of the power conversion system thereby making this filtering technique quite expensive to implement.
As opposed to the passive or active filtering techniques discussed above, which compensate for harmonic currents by either shunting or supplying the harmonic currents, respectively, the wave shaping technique attempts to draw from the utility system a current that is sinusoidal at the fundamental frequency (60 Hz in the U.S.). Commonly, this technique includes a switch-mode interface consisting of six control switches such as power transistors or gate-turn-off thyristors (GTO), each with a diode in antiparallel. This is a bidirectional current (power) interface, and permits the line currents drawn from or supplied to the utility system to be actively shaped to be sinusoidal. In addition, as opposed to the passive and active filters discussed above, this type of power conversion interface allows the DC voltage on the DC power system to be regulated at any desired value (greater than the peak line to line voltage). However, due to the sixswitch topology of this interface, and since power flow is typically only in one direction, that is, the switch mode interface is either functioning as a converter or an inverter, this type of interface is considered too expensive to implement.
Modulation of the currents on the DC side, at a harmonic frequency, and injecting the modulated current back into the AC side has been demonstrated in the prior art. However, in the prior art, separate sources for generating the harmonic current are used, and they do not use the direct current. In order to obtain a pure DC current, the prior art devices need large, costly inductors in the DC system, and they require an isolation transformer between the utility system and the DC system for operation and injection of the current which also raises the cost.
Examples of two types of systems that provide a harmonic distortion reduction using injection of a harmonic current are the A. Ametani, entitled "Generalized Method of Harmonic Reduction AC-DC Converters by Harmonic Current Injection", Proc. IEE Vol. 119, No. 7, July 1972, pages 857-864, and an article by B. M. Bird, J. F. Marsh and P. R. McLellan entitled "Harmonic Reduction in Multiplex Convertors by Triple-Frequency Current Injection" Proc. IEE, Vol. 116, No. 10, October 1969, pages 1730-1734.
In the Ametani article, current sources are used as shown in FIG. 2 for generating the harmonic current and in Section 3.1, FIG. 9, schematic representations of the applications are shown. In each instance, a separate source for generating the harmonic current that is injected is provided.
An injection of a harmonic current is also shown in the Bird et. al. reference, and here, too, a separate current source for generating the harmonic current is used. The prior art systems do not provide a controlled or regulated DC output, as does the present device, and the prior art devices required transformers for injection of the current for reducing harmonic distortion.
In the prior art, for the generation of harmonic currents a 4-quadrant converter will be required whose output voltage as well as current is alternating at the harmonic frequency. In the present device, the function of harmonic current generation is combined with the DC output regulation. As a consequence, only one quadrant converters are needed, whose output voltage and current both are DC, and always one polarity.